Ozone is a powerful oxidant which may be used as a disinfectant, bleach, or a reactant for an oxidation process. Ozone is easily detected by its pungent odor around electrical devices, especially photocopy machines. Industrial uses of ozone include; treatment of public water supplies (in 1990 there were 2000 plants worldwide, 20 in the U.S. and 50 in Canada built between 1970 and 1990 using ozone); treatment of industrial and municipal waste; silicon chip production; pharmaceuticals; and bleaching of fibers, fabric, paper pulp, and liquids. Being an oxidant, ozone may also be used as a bleach for cotton, paper, pulp, or liquids, or it may be used to treat waste streams which contain organic matter or require disinfection.
For many of these large quantity uses, excess ozone is present with water vapor. With such uses, ozone may replace chlorine as a bleach and mitigate the problems of chlor-carbon or organochlorine compounds remaining in the water, aerated, or formed during combustion of chlor-carbon waste.
For example, traditional chlorine bleaching creates wastewater containing hazardous organic compounds. In the wastewater stream, chlorine binds to organic matter to produce chemicals called organochlorines almost all of which are foreign in nature. These organochlorine compounds, which are extremely harmful, are very stable in the environment. Organochlorines bioaccumulate so they are stored in the body fat of humans and animals. Some of the known effects of organochlorines include hormonal disruption, infertility and lowered sperm counts, behavioral changes and damage to the skin, liver and kidneys. One of the best studied organochlorines, tetrachlorodiebenzo-p-dioxin or Dioxin, is a known carcinogen and thought to be an endocrine disrupter.
Use of chlorine bleaching in paper production results in hundreds of thousands of pounds of organochlorines being dumped into U.S. rivers annually. Studies have measured one hundred seventy seven different organochlorines in human populations throughout the U.S. and Canada.
Unlike the traditional chlorine bleaching methods used in pulp bleaching and other industrial uses, ozone bleaching offers the ability to close a mills water system and produce totally chlorine-free pulp.
Although ozone is important in a wide range of industrial and municipal processes, it is a toxic chemical. Ozone has a threshold limiting value of 0.1 ppm, so that the production or use of this chemical requires destruction of any excess or residual ozone. Ozone bearing tail gases from industrial processes and indoor and aircraft contamination with ozone require treatment.
Unlike organochlorines, destruction of ozone is accomplished by heating vapors containing ozone to about 316.degree. C. for several seconds. Utilizing this method, approximately 65% of the energy for heating can be recovered by heat exchangers. However, such a system has a high operating cost due to the relatively high temperatures required by such a system.
Unlike industrial and municipal uses, low levels of ozone, as in aircraft cabins, may be removed by passing the ozone through an activated carbon filter or a filter with a catalyst. These filters must be periodically replaced because the carbon is consumed and the catalyst becomes poisoned. A useful catalyst for ozone treatment by filtration is "hopcalcite", which is about 80% MnO.sub.2, 0.2% Cu, and the residual is Li.sub.2 O and K.sub.2 O. Other catalysts for ozone, as given in the literature, are different transition metals, silver, and the noble metals.
Studies show that some ozone can be stored at low temperatures in a zeolite, and later destroyed at higher temperatures. Metal catalysts used with zeolites have been used to promote the decomposition of ozone. However, the metal catalysts are most often transition metals, noble gases or heavy metals which raises concerns in regard to health and costs.
Ozone may also be destroyed at somewhat lower temperatures by passing the vapors over hopcalcite, certain other metals, activated carbon, or silicate compounds such as zeolites, sand or other mineral matter. These compounds act as catalysts for the decomposition of ozone, but unfortunately the catalysts are deactivated by the adsorption of water vapor on the catalyst surface.
Studies which consider the effect of water vapor in a gas stream indicate that the efficiency of zeolite based systems is decreased as water is adsorbed. Zeolites, which are molecular sieves, are excellent desiccants but must be dehydrated to reactivate the bed. Dehydration requires that the reactor containing the zeolite be heated to volatilize the water, and then cooled to return to service. Thus the treatment is a batch process and continuous treatment requires several reactors. These reactor beds commonly contain a costly transition metal with some losses due to attrition and venting to the outside air.
In the situation of industrial and municipal uses, excess ozone is usually present with water vapor, making decomposition of ozone by these above referenced methods difficult. The ubiquitous water vapor results in deactivation of the catalyst. The catalyst may be regenerated by: 1) heating, or 2) using an electrical field as disclosed in U.S. Pat. No. 4,101,296. If regeneration is by heating to desorb the water, the destruction of ozone is a batch process with a cycle of destruction of ozone until the adsorption of water deactivates the catalyst bed, then desorption of the water by heating, followed by cooling of the bed for regeneration and reuse.
It is known that zeolites are extremely effective at decomposing ozone without a metal catalyst in the absence of water or water vapor. However, as previously stated, it is also known that the adsorption of water vapor renders the zeolites inactive for the destruction of ozone, and reactivation requires the zeolite to be dried. An approach to this problem has been to use a hydrophobic zeolite, as described in European Application No. 88301002.7. However, hydrophobic zeolites are considerably more expensive than hydrophilic zeolites, and are less efficient than hydrophilic zeolites in ozone destruction. Therefore, there is a need to improve techniques for using zeolites in destroying ozone, especially in situations where the ozone is present in water or water vapor.